Why Do ACL Injuries Occur in Female Athletes?

Why Do ACL Injuries Occur in Female Athletes?

A breakdown of what causes ACL injuries in female athletes.

No injury strikes more fear into the hearts of athletes and coaches than an ACL tear. ACL injuries are especially devastating because of the arduous rehabilitation process, the difficulty in regaining pre-injury competitive levels, and the increased risk for arthritis that occursdue to limited mobility. Unfortunately they’re all too common. I remember sitting in a room with 10 of my football teammates in college and realizing that I was just one of two people in the entire room that had never experienced an ACL tear. And yet, despite being prevalent amongst men, they’re seemingly ubiquitous amongst female athletes.

Ask Tom Brady about ACL Injuries…

Here are some staggering facts concerning female ACL injuries:

Women are two to four times more likely to tear their ACL than male counterparts involved in the same sport (1).

Women are three times as likely to experience a noncontact ACL tear (1).

75% of women who had previously experienced an ACL injury still experience limitations 12 years later (2).

The data is intimidating, which is why it’s important we understand what mechanisms predispose females to a greater incidence of ACL tears, and what measures, if any, can be taken to help reduce risk.

What is the ACL and How Does it Get Hurt?
If you’ve read my other articles on OneResult, you’ll realize I’m kind of a geek when it comes to anatomy. But the more knowledgeable you are about your body, the better you’ll be at training effectively and reducing injury. The ACL, or anterior cruciate ligament, is one of the four main ligaments that exist in the knee and runs from theback/outside portion (posterior-lateral) of the femur to the front/middle portion (anterior-medial) of the tibia.

The ACL prevents forward motion of the tibia in relationship to the femur. Typically, ACL failure occurs in activities that require sudden deceleration while cutting or pivoting, or landing from a jump (so sports like soccer, basketball and tennis are at a high risk). The valgus position of the knee (think collapsing of the knee inward) following these movements is thought to be a leading cause of ACL injury, as they increase relative strain on the ligament.

Picture of Knee Valgus

Additionally, landing on the heels has been shown to increase the risk for ACL rupture by causing ground forces to travel up to the knee, causing forward sliding on the tibia relative to the femur.This places the burden of restraint on the ACL.
So now that we know more about the ACL and how you tear it, it makes sense to discuss some of the uncontrollable and controllable risk factors.

Intrinsic versus Extrinsic Risk Factors
There are unfortunately some risk factors that cannot be controlled when it comes to ACL injuries. Known as intrinsic risk factors, they include estrogen versus progesterone levels, intercondylar notch width, and the Q-angle of the knee. Want an English translation? Read on.

Sex hormones released when a female athlete is menstruating may affect the mechanical properties of the ACL, making the ligament more likely to tear depending on the dominant hormone present (5). The more estrogen the athlete has, the more likely the chance of injury. Additionally, the width of the intercondylar notch of the knee (seen below) is thought to influence the ability of the ACL to respond to loading and may also help explain the sex-based disparity in ACL injuries (4). A smaller notch has been hypothesized to cause impingement on the ACL, increasing the likelihood the ligament will tear during cutting or pivoting (3,4) Lastly, an excessive Q-angle (see below), can cause internal rotation, knock knees, and an externally rotated tibia and leads to an increased risk for injury.

The intercondylar notch and the Q angle

While there are factors out of our control when it comes to ACL tears, there are also factors we have an influence over.

Neuromuscular Recruitment
Exposure of the ACL to 3-dimensional forces during landing and twisting without proper dynamic stabilization by the knee flexors, primarily the hamstrings and calf, increases the torque experienced in the knee. Females typically rely more heavily on their quads in response to forward motion of the tibia, increasing the risk of ACL trauma (more on this in a bit).

Additionally, athletes with less ability to control the change in their upper bodies while performing a “high risk” maneuver are more susceptible to tearing the ACL. Therefore, athletes who have weak core strength will have poor upper body control, once again making them more susceptible to ACL tears.

Hamstring to Quadriceps Strength Ratio
The ratio between hamstring strength to quadriceps strength is a common test to assess the function of the knee and muscle balance. The ratio looks at eccentric (lengthening) hamstring strength versus concentric quadriceps strength. The normal ratio of H:Q strength is 50-80%, but as the ratio draws closer to 100%, the hamstrings increase their ability to stabilize the knee.
Highly developed quads can lead to decreased hamstring strength, increasing the risk of an ACL injury. Because women tend to have more quad strength than hamstring strength they’re at a higher risk than their male counterparts.

Working on H:Q Ratio with Glute Ham Raises

Fatigue
There have been many researchers that have speculated a connection between fatigue and increased risk for ACL injury, though very few published reports exist on the subject. Fatigue is thought to affect the motor recruitment pattern of the muscles supporting the knee. However, none of the studies seem to be conclusive. Women who are more highly trained may be more capable of controlling their bodies when their muscles become fatigued, but the jury’s still out as to whether fatigue actually causes ACL injuries.

Hopefully you now have a thorough understanding of the controllable risk factors females experience when it comes to ACL injury risk, so in part 2 we’ll address specific exercises that can help minimize these potential risk factors.